Ultrasonic and microwave intrusion alarm systems are known for detecting the presence of an intruder within a protected area and providing an alarm indication in response to such detection. Generally, a continuous acoustic or electromagnetic signal is transmitted into the protected area. The backscatter signal from objects in the protected zone is received; and any movement in the protected area produces a doppler shift in the received signal which is detected to produce an alarm.
Frequently, the transmitting and receiving transducers or antennas are contained within the same housing, and this arrangement is called a transceiver. A housing suitable for such a transceiver is shown in design application Ser. No. 657,709, filed Feb. 12, 1976, now U.S. Pat. Des. No. 243,513, and assigned to the same assignee as the present invention. Although transceivers offer a number of advantages such as ease of mounting, the need for only a single enclosure, and best averaged doppler frequency against an intruder walking in random directions in the protected area, transceivers have the disadvantage of being extremely sensitive to disturbances occurring very close to the transmitting and receiving elements. The "close-in" sensitivity varies approximately as 1/R.sup.4. Because of the rapid increase in system sensitivity as a target approaches the transceiver, a very small, close-in target will produce the same alarm signal level as a large target further away. As an example, a device which is designed to trigger an alarm when an intruder with a cross-section of approximately one square meter is viewed at a distance of 10 meters, will produce the same response to an insect having a cross-section of 10.sup.-6 square meters located 1/3 meter from the transceiver. This high close-in sensitivity can produce undesirable false alarms from an intruder-detection system.
One method commonly used to overcome such close-in sensitivity is to separate the transmitting and receiving transducers so that the close-in sensitivity is reduced to an acceptable level, due to the fact that when the close-in target is very near the transmitting transducer, it is not very near the receiving transducer. This method has the disadvantage of requiring larger transceiver enclosures which are more expensive and more difficult to conceal. Another method is to add a pair of baffles to the transceiver, one beside each transducer, to provide a "shadow" in the area of close-in sensitivity. This method has the disadvantage of lowering sensitivity in the direction of the original beam directions. Also, transceivers have been built with transducers which may be tilted and rotated to point towards any particular part of the protected area. The freedom of movement of the transducers in this type of transceiver is to allow the area of coverage of a transceiver to be varied without changing its mounting location; and the problem of close-in sensitivity of such a transceiver frequently is greater than when the transducers are parallel in orientation.